1. Field of the Invention
The present invention relates generally to alkali metal/oxygen electrochemical energy storage cells and in particular embodiments to secondary Li/Air battery cells and aqueous catholytes for use therein as well as to methods of operating said cells.
2. Related Art
The large free energy of the reaction between lithium and oxygen has attracted the interest of battery researchers for decades. At a nominal potential of about 3 volts, the theoretical specific energy for a Li/Air battery in a non-aqueous electrolyte is over 11,000 Wh/kg for the reaction forming Li2O2 (2Li+O2=Li2O2), and in aqueous electrolytes 5,000 Wh/kg for the reaction forming LiOH (Li+¼O2+½H2O=LiOH), both systems rivaling the energy density for hydrocarbon fuel cells and far exceeding lithium ion battery chemistry. Indeed, as evidenced by the development and commercial success of the Zn/Air battery, the high specific energy for metal/air chemistries has been long recognized. Li/Air chemistry, however, introduces additional challenges.
Today it is generally recognized that there are two basic approaches to Li/Air battery development depending on whether the electrolyte in contact with the cathode is aqueous or non-aqueous (i.e., aqueous Li/Air or non-aqueous Li/Air).
Abraham et al. were the first to describe a non-aqueous Li/Air battery using organic carbonate electrolytes commonly employed for lithium batteries. Thereafter, in U.S. Pat. Nos. 7,282,295 and 7,645,543 to Visco et al., for example, improved performance is described based on electrolyte formulations other than carbonates, and more generally electrolytes that, albeit unstable in contact with lithium, were enabled for use in a Li/Air cell because the lithium anode was isolated from contact with the electrolyte by a protective membrane architecture. Notwithstanding those improvements, non-aqueous Li/Air cells can be plagued by the formation of a copious amount of insoluble discharge product that can clog cathode pores, severely limiting both discharge capacity and cycling stability.
Relative to its non-aqueous counterpart, aqueous Li/Air has its own set of unique challenges. Most prominent of these is the reactivity of bare lithium metal in contact with water. As described in U.S. Pat. Nos. 7,645,543 and 7,282,295, for example, practical aqueous Li/Air batteries depend upon stabilizing the lithium anode (e.g., lithium metal) in the presence of water and oxygen. Protected lithium electrodes suitable for use in aqueous Li/Air batteries are described in U.S. Pat. Nos. 7,282,295 and 7,645,543, for example. The protected lithium electrodes have protective membranes and protective membrane architectures that are stable in water environments and are capable of discharging into aqueous catholytes. For cells employing a protected lithium electrode, the aqueous electrolyte in contact with the cathode does not contact the lithium anode, and for this reason is generally referred to herein and elsewhere as catholyte, and when the catholyte solvent system is primarily composed of water it is generally referred to as an aqueous catholyte.
Today it is generally accepted that in practice, at least for most applications, the lithium anode in an aqueous Li/Air battery cell must be protected against direct contact with the aqueous electrolyte.
For Li/Air, the type of catholyte (e.g., aqueous or non-aqueous) employed, and its particular formulation, is determinative of the chemistry taking place at the cathode, and ultimately overall cell performance. The development and search for improved aqueous catholytes for Li/Air batteries is not simple, the requirements are several-fold, and the results can often be unpredictable.